So a step by step guide for people in different scenarios (feel free to add to correct as needed):
If your new to Xbox or have no active subs from an old account:
1) You can buy upto 3 years of Gold/Standard Game Pass direct from MS or from a third party retailer that will sell you those cheaper 12 month codes (newegg and other sites have been mentioned in this thread - have a look around based on your region) - again any amount up to 3 years can be bought.
2) Go to
https://redeem.microsoft.com/ - sign in with your Xbox log in. Make sure you log in with the same email/pw as you would use for your xbox profile and not a work/school related one! If you have multiple accounts you can always sign in at xbox.com, check which xbox profile is connected to that account and then sign in at the account page.
3) Redeem your 3 codes that you purchased from the retailer into that website.
4) Go to
https://www.xbox.com/en-AU/live/gold/upgrade (this should recognise what country your in and take you there to your store front). Hit upgrade now for $1.
5) There will be a pop-up explaining that any exsisitng gold/gamepass time will be converted to ultimate. If you do not have a credit card on file, you will now need one added.
6) Time will now be converted to 3 years of Ultimate Game Pass.
7) Go to
https://account.microsoft.com/, go to the Services and Subscriptions tab, you will see that Ultimate Game Pass should be active and next charge should be 3 years from now. If you want to cancel the auto-renew, click manage next to that Game Pass Ultimate and then next to Recurring billing, click Change and then hit Turn Off Recurring billing.
If you already have some months of Gold/Game Pass - be careful buying the full 3 years, you likely will not be able to redeem 3 full years of codes. Any exsisting time you have prior to purchasing your codes is also going to be converted 1:1 and 3 years is the limit it will convert into - do not purchase more then you need.
If you have had Game Pass Ultimate previously but not currently - you can follow the guide above and the steps are the same but you will likely have to pay $15 to upgrade the gold to Game Pass Ultimate instead of the $1 teaser rate.
If you have Game Pass Ultimate currently and want to do this again - let your Game Pass Ultimate sub lapse and you can then do this again.